1. Field of the Invention
The present invention generally relates to an optical scanning device used in an image forming apparatus, such as a laser printer, a digital copier, a laser facsimile, a laser plotter and so forth, as an optical writing part or the like thereof.
2. Description of the Related Art
Such an optical scanning device has been known in which a beam emitted from a light source such as a semiconductor laser or the like is deflected by a light deflector such as a polygon mirror, the deflected beam is condensed toward a surface to be scanned such as a photosensitive body or the like by a scanning and imaging component so that a beam spot is formed thereby on the surface to be scanned, and the surface to be scanned is scanned by the beam spot.
In an image forming apparatus of an electrophotographic type employing such an optical scanning device, optical scanning of an electrically charged photosensitive body is performed with a beam modulated in accordance with image information by the optical scanning device. Thereby, an electrostatic latent image is formed on the photosensitive body. This latent image is visualized with toner by a developing device, is then transferred and fixed to a recording paper sheet or the like. Thus, a record image is obtained. However, in such an image forming part of the image forming apparatus, floating toner is generated, powder is generated from the recording paper sheet, and so forth. When such floating dust enters the optical scanning device, it adheres to the scanning and imaging component, light deflector and so forth. Thereby, the optical scanning cannot be performed properly. In order to solve this problem, the light source, light deflector, scanning and imaging component and so forth are enclosed by an optical housing, and, a light exit thereof through which a beam exits is sealed up by a transparent parallel plate for dustproof, in the related art.
Further, in such an optical scanning device, for the purpose of preventing generation of noise in a light deflector such as a polygon mirror or the like, stain of the deflection reflective surface of the light deflector, and so forth, the light deflector is covered by a housing or a cover, and a light entrance of a beam toward the light deflector and a light exit of the beam from the light deflector are sealed up by a transparent parallel plate for soundproof and dustproof.
However, when such a transparent parallel plate for soundproof and/or dustproof is provided for the optical housing covering the entirety of the optical scanning device, or, for a housing of the light deflector, ghost light is generated as a result of a beam reflected by the transparent parallel plate returns to the deflection reflective surface of the light deflector, astigmatic difference of the transparent parallel plate adversely affects the imaging performance of the scanning and imaging component, and so forth.
In order to solve these problems, the parallel plate for soundproof and dustproof is inclined with respect to the optical axis so that the beam reflected thereby is prevented from returning to the deflection reflective surface of the light deflector.
However, by simply inclining the parallel plate with respect to the optical axis, bending of scan line occurs on the surface to be scanned.
Further, it is preferable that a transmission optical component such as a transparent parallel plate is as small as possible. Further, unevenness in light intensity (shading) occurs on the surface to be scanned due to the transmission optical component, and, thereby, the record image is degraded.
The present invention has been devised in consideration of the above-described situation, and, an object of the present invention is to eliminate ghost light while dustproof and soundproof is achieved, and, also, to reduce bending of scan line.
Another object of the present invention is to reduce unevenness in light intensity (shading) on the surface to be scanned.
In order to achieve these objects, an optical scanning device according to the present invention includes:
a light deflector performing deflection scanning through reflection of a beam coming from a light source; and
at least one scanning and imaging component condensing the beam having undergone the deflection scanning performed by the light deflector toward a surface to be scanned,
these components being enclosed by an optical housing,
wherein:
the device further comprises two transmission optical components each having approximately planar surfaces on both sides thereof;
the two transmission optical components are disposed so as to have the at least one scanning and imaging component inserted therebetween;
one of the two transmission optical components (one nearer to the light deflector) is disposed at a light exit of a cover covering the light deflector;
the other of the two transmission optical components is disposed at a light exit of the optical housing;
the two transmission optical components are inclined in a same direction in a sub-scanning section including a straight line perpendicular to the surface to be scanned;
the transmission optical component disposed at the light exit of the cover of the light deflector is inclined with respect to the surface to be scanned in a deflection scanning plane;
the transmission optical component disposed at the light exit of the optical housing lies approximately in parallel to the surface to be scanned in the deflection scanning plane; and
the polarization directions of the beam directed to the light deflector are approximately perpendicular to the deflection scanning directions.
Thus, by configuring the optical scanning device such that the two transmission optical components disposed at the light exit of the cover covering the light deflector and at the light exit of the optical housing are inclined in the same direction in the sub-scanning section including the straight line perpendicular to the surface to be scanned, light reflected by the transmission optical components is prevented from returning to any of the deflection reflective surface of the light deflector and the scanning and imaging component. Accordingly, it is possible to prevent ghost light from being generated while dustproof and soundproof of the optical scanning device are rendered. Further, it is possible to well correct bending of scan line on the surface to be scanned.
As mentioned above, the transmission optical component disposed at the light exit of the cover of the light deflector is inclined with respect to the surface to be scanned in the deflection scanning plane; the transmission optical component disposed at the light exit of the optical housing lies approximately in parallel to the surface to be scanned in the deflection scanning plane; and the polarization directions of the beam directed to the light deflector are approximately perpendicular to the deflection scanning directions. Thereby, characteristics of reflectivity of the light deflector for respective field angles and a characteristic of transmittance of the transmission optical component, which is the one nearer to the light deflector, are cancelled out by one another. Accordingly, it is possible to effectively reduce shading (unevenness in light intensity on the surface to be scanned). Further, it is possible to reduce both transmission optical components in size as a result of the transmission optical components being disposed as mentioned above. Accordingly, it is possible to improve the optical scanning device in performance and in costs.
Further, the following conditional formula may be satisfied:
2.5 less than (d2xc3x97sin xcex82)/(d1xc3x97sin xcex81xc3x97|xcex2|) less than 7.0 
where:
xcex81 denotes a rotation angle of the transmission optical component having the approximately planar surfaces on both sides thereof, which is the one nearer to the light deflector;
xcex82 denotes a rotation angle of the other transmission optical component having the approximately planar surfaces on both sides thereof;
xcex2 denotes a sub-scanning lateral magnification between the light deflector and the surface to be scanned;
d1 denotes a thickness of the transmission optical component having the approximately planar surfaces on both sides thereof, which is the one nearer to the light deflector; and
d2 denotes a thickens of the other transmission optical component having the approximately planar surfaces on both sides thereof.
Thereby, it is possible to reduce the bending of scan line on the surface to be scanned effectively.
In the optical scanning device according to the present invention, mainly a semiconductor laser (LD) or the like is used as the light source. However, a light emitting diode (LED) or the like may be used alternatively. The beam coming from the light source is coupled by a coupling lens or the like, and, thereby, the beam becomes a parallel beam, divergent beam or a convergent beam, which is then incident on the light deflector.
As the light deflector, a polygon mirror having a plurality of deflection reflective surfaces, a rotational single-surface mirror such as a pyramidal mirror, a tenon-type mirror or the like, or a swinging mirror such as a galvano mirror may be used.
Further, in order to correct so-called surface inclination, the coupled beam may be condensed only along the sub-scanning directions by an optical system such as a cylindrical lens disposed on the light path between the coupling lens and light deflector so that the beam is used for imaging a line image long along the main scanning directions at a position of the deflection reflective surface of the light deflector.
The scanning and imaging component is used for imaging a minute beam spot on the surface to be scanned with the beam having undergone deflection scanning performed by the light deflector, and includes at least one lens, mirror or the like having a power along the main scanning directions and/or sub-scanning directions. When the light deflector is of a rotational member such as a polygon mirror, a scanning and imaging lens such as an fxcex8 lens or the like, fxcex8 mirror or the like may be preferably used. When the light deflector is a swinging mirror, a scanning and imaging lens such as an f sin xcex8 lens or the like may be used. By using such a scanning and imaging component, it is possible to make the main scanning speed uniform.
Further, when the coupled beam is condensed only along the sub-scanning directions and is used for imaging a line image long along the main scanning directions at the position of the deflection reflective surface of the light deflector for the purpose of correcting surface inclination as mentioned above, the scanning and imaging component has a function of causing the position of the deflection reflective surface and the position of the surface to be scanned to be approximately conjugate in geometrical optics for the sub-scanning directions.
The transmission optical component mounted at the light exit of the cover covering the light deflector and the transmission optical component mounted at the light exit of the optical housing may be parallel plates made of transparent glass or resin each having approximately planar surfaces on both sides thereof preferably, and function as dustproof and soundproof members. However, they are not limited to parallel plates, and may be transparent plates each having a slight angle between both sides thereof.
Other objects and further features of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.